1. Field of the Invention
The present invention relates to an ignition system, and more particularly, to an injector for such ignition systems.
2. Description of the Prior Art
It has been found that dependable ignition using conventional aircraft engine ignition systems is not possible under certain situations. Of particular difficulty is to attempt to ignite a small engine at very low speed using large flow number or air blast fuel injectors. Small engines cannot produce a significant combustor pressure drop during cranking conditions and, therefore, atomization of fuel by the main fuel injector is poor.
Large flow number injectors are sized for optimum performance at full power. Fuel flows during the starting mode are not sufficient to pressurize the injectors at the starting mode flow rates and thus atomization of the fuel is inadequate, resulting in difficulty.
So-called hot starts can result if the fuel flow rate is increased at such low speed conditions. In order to solve the above problem, very small pressure atomizers, known as primary injectors, are used in association with larger secondary injectors. The small size of these primary injectors produces a greater fuel pressure at low speed, allowing for better atomization of the fuel.
The disadvantage with these small primary injectors is their tendency to become contaminated owing to the very small orifice sizes. This requires increased maintenance in the field. In addition, a second fuel manifold and fuel flow divider valves add to the complexity of the system and to the cost. At high altitude and in conditions where flame-out of the combustor might be more prevalent, requires that the primary injectors be continuously operating to inject fuel into the combustion chamber which, in fact, diverts fuel from the secondary fuel injectors to thereby hamper the cooling of the secondary injectors.
So-called torch igniters utilize a small primary injector in close proximity to the igniter, thus eliminating the requirement for a large number of small injectors. However, these primary injectors still have the problem of contamination in view of their very small orifice sizes. In order to keep the injector cool, it must be operated throughout the entire engine cycle.
Although torch igniters can solve some problems, particularly of ignition during low speed cranking conditions, their performance can still suffer at high altitudes when it is required to reignite after a flame-out, because the air flow rates and the combustor pressure drops are much greater.